Masking composition for galvanized metal



3,398,010 MASKING COMPOSITION FOR GALVANIZED METAL Mahlon A. Harvey, Parrna, Ohio, and Lawrence E. Helwig, Glenshaw, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Aug. 17, 1964, Ser. No. 390,237

4 Claims. (Cl. 1175.5)

ABSTRACT OF THE DISCLOSURE A method of making a ferrous metal article having only a portion galvanized which involves applying as a shielding coating to a ferrous metal article a mixture comprising boric acid, kaolin and attapulgite clays, thereafter galvanizing the unshielded portion and removing the shielding coating after galvanizing. The application of a coating of boric acid and clay in specified ranges also improves the corrosion resistance of the surface to which it has been applied.

This invention relates to the manufacture of galvanized ferrous metal. More particularly, the invention is directed to a method of galvanizing only a portion of a ferrous metal article, and to an improved galvanized article which is produced and to a novel shielding composition. The invention is particularly useful for manufacturing galvanized steel strip which has a galvanized coating on one side only. The terms ferrous and ferrous metal as used herein refer to iron and iron alloys.

Galvanized steel is generally used in applications Where exposure to corrosive elements may be severe. It has been used extensively in the automotive industry to improve corrosion resistance of exposed parts of a vehicle. For many applications, however, it is necessary to weld galvanized metal parts, e.g. by resistance welding, and the use of galvanized steel has been limited because the zinc coating adheres to the Welding electrodes. It is desirable, therefore, to use galvanized material that is coated on one side only to facilitate welding as well as to decrease costs where it is unnecessary to have all surfaces coated for protection against corrosion.

The present invention provides a method of making galvanized ferrous metal articles which have only a portion of the article galvanized and which imparts a degree of corrosion resistance to the ungalvanized portion as well. According to the invention, a ferrous metal article is provided with a shielding coating which consists essentially of a mixture of boric acid and a clay selected from the group consisting of kaolin and attapulgite. After coating, the material is galvanized by immersing it in molten spelter, i.e. molten zinc or zinc alloy, and the portion of the article which was shielded prior to galvanizing remains unglavanized. The shielding material is then removed and the bare metal surface which is exposed exhibits better resistance to corrosion than untreated metal.

As mentioned above, the shielding-composition of our invention consists essentially of a mixture of boric acid and at least one clay from the group consisting of kaolin and attapulgite. Attapulgite and kaolin are two forms of clay which contain aluminum silicate. Attapulgite is a clay mineral believed to have the formula s s zz 4 z Kaolin is a hydrous silicate of aluminum in the form of a fine white clay and includes the kaolinite group of minerals. The mixture of boric acid and clay, according to the preferred embodiment of the invention, contains 4 to 20 percent by weight boric acid and 80 to 96 percent by weight clay. The clay fraction of the mixture can connited States Patent sist of entirely kaolin or attapulgite, depending upon the specific properties desired in the final coating. Where removal after galvanizing is not necessary, and abrasion resistance is of prime concern the clay content of the coating composition may consist of kaolin alone. Where removal after galvanizing of the shielding material is important and the abrasion resistance is relatively unimportant (e.g., where no foreign bodies contact the claycoated surface) the clay content of the mixture may consist entirely of attapulgite clay.

The material used to shield the surface during galvanizing must be present as a continuous Ifilm over the entire surface. If discontinuities are present, zinc will adhere to the exposed areas during galvanizing and the resulting zinc deposits would be difiicult, if not impossible, to remove. Where continuously processed steel strip is treated for zinc coating on one side only, the shielding coating must be sufliciently adherent so it wont come off when the strip is passed over metal rolls in the processing line. However, the coating material must also be capable of easy and complete removal after galvanizing. The coating composition according to our invention has been found to be satisfactory in all respects and provides the additional advantage of improving the corrosion resistance of the ungalvanized surface.

The following example illustrates the preferred embodiment of the invention as applied to ferrous metal sheet or strip.

A steel strip is passed through a tank containing a cleaning solution such as a suitable detergent. After cleaning, a shielding coating comprising a mixture of boric acid, kaolin and/or attapulgite is applied to the surface of the strip which is not to be galvanized. The coating may be applied as an aqueous slurry, preferably by passing over a roll partially immersed in the slurry. The coating, after application to the strip surface, is partially dried to remove most of the uncombined water by passing the strip through a heating chamber which may, for example, be gas fired or electrically heated. Upon leaving the heating chamber, the strip is advantageously maintained under nonoxidizing conditions until it enters an annealing furnace where the metal is heated to annealing temperature, e.g. about 1300 F., in a reducing atmosphere such as hydrogen gas and drying is com pleted with the removal of uncombined water. The strip is conveyed also under nonoxidizing conditions, to a zinc-coating tank where it is submerged in a bath of molten spelter from which it emerges with a zinc coating on the unshielded surface. The partly galvanized strip can then be passed through a water tank where the protective or shielding coating can be removed by any suitable means such as by a high pressure water stream, or a rotary brush. Brushing and washing with water is preferred and can easily remove the protective-coating composition from the strip to give a clean zinc-free surface. The strip can then be dried by passing through a heating chamber, which may be similar to that previously described, and thereafter may be coiled.

It is preferred to apply the shielding composition to the ferrous metal surface as an aqueous slurry. The quantity of water used to form the slurry may vary depending upon the method of applying the shielding coating to the article. However, although the amount of water used in a slurry depends on the coating facilities, a solids-towater ratio of about 0.5 to about 0.6 produces the desired fluidity and is preferred. Where the aqueous slurry is applied by spraying, the slurry must contain more water than if the composition is applied by roller coating or brushing. We have found that the amount of water in the slurry is not critical but that for continuous strip processing, the solid residue after drying should provide a coating thickness within the range of about 0.6 to 1.1 mils. A

coating thickness of 0.75 mil is preferred and is suitable for most applications. If the coating thickness exceeds 1.1 mils, the coating tends to flake oif the strip as the strip contacts the various rollers in the processing line. If the coating thickness is less than 0.6 mil, it is diflicult to insure a continuous film of the composition on the surface. The mixture itself has good stability and has been found to be fluid and useful after 25 days storage in a sealed container.

The abrasion resistance of the coating and the effectiveness of the composition as a shield in preventing molten zinc from contacting the ferrous metal surface has been found to increase as the quantity of boric acid and kaolin in the mixture increase. However, the ease with which the coating may be removed or its scrubbability is correspondingly impaired as the abrasion resistance and shielding properties improve. If little or no attapulgite clay is used, the resulting coating is extremely difficult to remove; whereas, with lesser amounts of boric acid and kalin the coating is extremely easy to remove but exhibits less abrasion resistance and shielding. We have found that to provide a protective shield suiltable for use in partial-surface galvanizing operations, the kaolin should be present in an amount of from about 60 to 70 percent by weight of the total solids in the slurry and total amount of kaolin and boric acid should not exceed 75 percent of the solids content, the remainder being attapulgite clay.

The results of a series of tests to determine abrasion resistance, shielding effectiveness, and scrubbability of samples coated with various combinations of boric acid, kaolin and attapulgite are presented in Table I. As is seen, excellent results in all categories are obtained only when a mixture of boric acid, attapulgite and kaolin is used; but, mixtures of boric acid and one of the clays exhibit some good properties which may be suflicient for some purposes. As discussed above, the selection of a specific composition depends to some extent on the particular treatment involved.

As can be seen from the above data, the samples that had been coated with the shielding composition exhibited corrosion resistance superior to that of an identical steel sample (Sample F) that had not been provided with the coating. The reason for the improved corrosion resistance is not fully known. However, it is thought that some compound is present on the ferrous metal surface after removal of the coating which effectively acts to impede corrosion of the surface. Inspection of the surfaces reveals some discoloration indicating the presence of a surface film. It is also noted that samples coated with a mixture of boric acid and only one clay (Samples A and B) yielded a surface after the coating was removed by washing, which had considerably better corrosion resistance than the control sample (F) of the same steel which had not been coated. Sample A which was a mixture of boric acid and attapulgite demonstrated astonishingly superior corrosion resistance.

We claim:

1. A process for producing a ferrous metal article having only a portion galvanized, comprising applying as a shielding coating to a portion of the article not to be galvanized a mixture consisting essentially of kaolin, boric acid and attapulgite, wherein the kaolin is present in an amount of from 60 to 70% by weight of total solids in the coating mixture and the total weight of kaolin and boric acid does not exceed 75% of the total solids, the remainder being essentially attapulgite clay, galvanizing the unshielded portion by immersing the article in molten spelter and removing the shielding coating after galvanizing.

2. A process according to claim 1 wherein said shielding coating is applied as an aqueous slurry and dried.

3. A process according to claim 1 wherein a shielding coating between about 0.6 and about 1.1 mils thick is provided on the ferrous metal article.

4. A shielding coating composition consisting essentially of a mixture of boric acid, kaolin and attapulgite, wherein the kaolin is present in an amount of from about TABLE I Composition of Solid Components Evaluation 01 Coated Surface Mix No.

Boric Acid Kaolin Attapulgite Abrasion Shielding Scrubbe- Resistance bihty 21.7 78.3 Fair Fair Excellent. 13. 4 86. 6 Poor d Do. 13. 4 Excellent Excellent Poor.

5. 9 d Do. 13. 4 26. 4 11. 1 27. 2 5. 3 26. 3 9. 5 27. 6

The results of a series of tests to illustrate the relative corrosion resistance of some of the samples listed in Table I are reported in Table II below. Samples B, C, D and B were coated with mixtures corresponding to mix Nos. 4, 5, 7 and 8 respectively, from Table 1. Sample A was a mixture of boric acid and attapulgite and is included with sample B to show that it is not necessary to have both clays in the mixture to realize improved corrosion resistance. After the mixtures had been washed off, the samples were tested for corrosion resistance in salt spray and humid storage tanks.

TABLE II Corrosion Teststime for 01' 1 Mixture of 7.5% Boric Acid and 92.5% Attapulgite (no kaolin). 2 Control (cold-rolled steel).

60 to 70% by weight and the total weight of solids and the total weight of kaolin and boric acid does not exceed 75%; the remainder being essentially attapulgite clay.

References Cited UNITED STATES PATENTS 826,157 7/1906 Fowler 148-6.14 2,005,780 6/ 1935 Gravell 1486.17 2,493,516 1/1950 Zimmer et al. 1486.14 2,894,850 7/ 1959 Greene et al 1175.5 3,048,495 8/1962 Petkus et al. 117169 3,104,993 9/1963 Sievert et al. 1l7-5.5 3,121,019 2/1964 Blay 117--5.5 3,149,987 9/ 1964 Grandall 1175.5 3,151,002 9/1964 Benz et a1 148-63 3,177,085 4/1965 Adams 1175.5 3,181,963 5/1965 Cook et al 1175.5

FOREIGN PATENTS 15,566 6/1915 Great Britain. 963,450 7/ 1964 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

A. M. GRIMALDI, Assistant Examiner. 

